Musnad Ahmad 30:2320

Hassan ibn Musa narrated to us, he said: Shayban narrated to us, from Yahya ibn Abi Kathir, he said: Abu Salamah informed me that Abu Hurairah informed him that the Messenger of Allah, peace and blessings of Allah be upon him, said: "Whoever stands [in prayer] during Ramadan out of faith and seeking reward, his past sins will be forgiven. And whoever stands [in prayer] on the Night of Decree out of faith and seeking reward, his past sins will be forgiven."